A hard-disk drive (HDD) is a non-volatile storage device that is housed in a protective enclosure and stores digitally encoded data on one or more circular disks having magnetic surfaces. When an HDD is in operation, each magnetic-recording disk is rapidly rotated by a spindle system. Data is read from and written to a magnetic-recording disk using a read/write head that is positioned over a specific location of a disk by an actuator.
A read/write head uses a magnetic field to read data from and write data to the surface of a magnetic-recording disk. Write heads make use of the electricity flowing through a coil, which produces a magnetic field. Electrical pulses are sent to the write head, with different patterns of positive and negative currents. The current in the coil of the write head induces a magnetic field across the gap between the head and the magnetic disk, which in turn magnetizes a small area on the recording medium.
While increasing areal density, a measure of the quantity of information bits that can be stored on a given area of disk surface, is one of the ever-present holy grails of HDD design evolution, the cost per unit of storage is also an ever-present constraint associated with HDD development. The cost per storage capacity, or price per capacity from a consumer's standpoint, is especially important in large data storage scenarios, such as with archival and backup storage in which large amounts of data are stored but infrequently accessed.